(1) Field of the Invention
The present invention relates to a coded transmission system wherein each bit of transmitted data having a logical value is coded depending on a coded result of a preceding bit. The present invention further relates to a line coding apparatus and a line decoding apparatus used in the above coded transmission system.
(2) Description of the Related Art
Line coding systems wherein each bit of data having a logical value is coded depending on a coded result of a preceding bit e.g., the bipolar coding system, or the CMI (coded mark inversion) coding system, are used for line coding of transmission of a logical value.
In the bipolar coding system, each bit of data having a logical value "0" is coded to a zero level, and each bit of the data having a logical value "1" is alternatively coded to a positive level and a negative level, i.e., each bit of the data having a logical value "1" is coded to the positive level when a preceding bit of the data having the logical value "1" is coded to the negative level, and each bit of the data having a logical value "1" is coded to the negative level when a precedicing bit of the data having the logical value "1" is coded to the positive level. In the CMI coding system, each bit of data having a logical value "0" is coded by changing the coded signal level from a zero ("0") level to a positive ("1") level in the center phase of one cycle, and each bit of the data having a logical value " 1" is alternatively coded to the positive ("1") level and the zero ("0") level, i.e., each bit of the data having a logical value "1" is coded to the positive ("1") level when a preceding bit of the data having the logical value "1" is coded to the zero ("0") level, and each bit of the data having a logical value "1" is coded to the zero ("0") level a preceding bit of the data having the logical value "1" is coded to the positive ("1") level. In both coding systems, a code which does not accord with the above rules, is determined to be a violation of the rules, respectively, in the receiver side. Namely, in the above coding systems, a line decoder in the receiver side is required to memorize a coded mark (level) of a preceding bit of data having a logical value "1".
In the recent data transmission systems, high speed transmission is required. In particular, in data processing systems wherein a plurality of data processing apparatuses are connected with each other by data transmission lines, data is transmitted asynchronously between the apparatuses. In such asynchronous data transmission systems, use of the CMI coding is considered to be advantageous because the CMI coded signal can be relatively easily synchronized in the receiver side.
In the above asynchronous transmission systems wherein a plurality of data processing apparatuses are connected with each other by data transmission lines, a receiver in each of the data processing apparatuses or data transmission apparatuses receives coded signals which are transmitted from a plurality of data processing apparatuses or data transmission apparatuses. When the above line coding systems wherein each bit of data is coded depending on a coded result of a preceding bit, like the bipolar coding system, or the CMI coding system, are used for line coding in the above data transmission system, each receiver is required to memorize a coded mark (level) of a preceding bit of data having a logical value "1" for each of a plurality of data processing apparatuses or data transmission apparatuses from which coded signals are asynchronously transmitted to the receiver. However, such memorizing is very bothersome, and it is almost impossible.
In addition, in the above asynchronous data transmission systems, a synchronization pattern is transmitted preceding a real message portion. FIG. 1 shows an example of a beginning portion of a conventional asynchronous transmission signal using the CMI coding. In the example of FIG. 1, a synchronization pattern which is comprised of three synchronization bits "000" is transmitted preceding a real message portion, i.e., the CMI-coded level (010101) is transmitted preceding a CMI-coded real message portion. However, each bit of the synchronization pattern may be recognized erroneously because the receiver is generally not synchronized with the asynchronous transmission signal when receiving the synchronization pattern. FIG. 2 shows an example of an erroneously recognized beginning portion of a conventional asynchronous transmission signal using the CMI coding. In the example of FIG. 2, the code (01) of the second bit of the synchronization pattern is erroneously recognized as (11), i.e., the second bit of the synchronization pattern is erroneously recognized as "1". Responding to the recognition of the (11), the receiver memorizes the reception of the code (11), and when a bit of the real message having a logical value "1" is transmitted to the receiver with coded as (11) following the erroneous recognition, the receiver determines the received code (11) as a violation of the rule of the CMI coding.
Namely, in the above asynchronous data transmission systems wherein each bit of data is coded depending on a coded result of a preceding bit, as the above CMI coding, the coded signal may not be correctly decoded in the receiver side.